The discovery of carbon nanotubes has attracted enormous attention over the past decade due to their potential significance in nanoelectronic devices (S. Iijima, Nature vol. 354, 56-58 (1991)). Micro and nano tubules produced from amphiphilic lipids have also captured the imagination of scientists in disciplines from biology through material science to chemistry and physics (J. M. Schnur, Science 262:1669-1676 (1993)). Tubules of this type have promises as advanced materials in a plethora of applications ranging from small molecular wires, to drug encapsulation, to biosensors. However, to date only a few classes of lipids, nearly all of which are chiral, are shown to have the capacity to form tubular structures under controlled conditions. Schnur et al., in U.S. Pat. No. 4,887,501, disclosed the use of phosphoglycerides derived from diacetylene carboxylic acids which self-assembled into tubular microstructures upon a change in solvent polarity. The tubes so formed were generally not uniform in size: although specific conditions yielded narrow distributions in diameter, tube lengths varied considerably.
There remains a need for a method of preparing tubules of uniform diameter and length. The difficulty in preparing optically active phospholipid variants is another major obstacle to the use of typical lipids and phospholipid analogues in the fabrication of lipid helices and tubules.
Various attempts have been made to overcome these problems by chemical modification of amphiphilic diacetylene lipids. Schoen et al. have discussed method of making lipid tubules composed of chiral diacetylenic phosphocholine by a cooling process (U.S. Pat. No. 4,990,291). The diacetylenic phosphocholines have distinctly different endothermic and exothermic transition temperatures. Lipid tubules can be formed by hydrating a diacetylenic phosphocholine at a temperature above its endothermic transition temperature then slowly lowering the temperature. Unlike spherical liposomes, lipid tubules reflect the chiral nature of the lipids used to form them. This chirality in molecular packing is reflected in the helical structures, often visible in electron microscopy images of the tubules, and in large peaks observed in their circular dichroism (CD) spectra. The helicity and the CD spectra of the tubules change handedness when the opposite enantiomer lipid is used.
Tubules were observed by Schoen and Yager (Mol. Cryst. Liq. Cryst. vol. 106, 371 (1984)) as having assembled in water from liposomes of the two-chain chiral lipid diacetylene, 1,2-bis(tricosa-10,12-diynoyl)-sn-glycero-3-phosphocholine (“DC8,9PC”). Tubules formed from DC8,9PC have an average diameter of 0.5 μm and lengths which range from 50 to 200 μm. The size and stability of these tubules were sensitive to preparation conditions and thermal history, resulting in a non-homogenous preparation. Other work with chiral lipids bearing two diacetylenic chains has demonstrated that it is difficult to generate uniform nanotube structures from these precursors (see, e.g., Thomas et al., Science vol. 267, 1635 (1995); Spector et al., Nano Letters vol. 1, 375 (1984); Wand et al., Langmuir vol. 15, 6135 (1999); Svenson et al., Langmuir vol. 15, 4464 (1999); Seddon et al., Angew. Chem. Int. Ed. vol. 41, 2988 (2002); and Thomas et al., J. Am. Chem. Soc. vol. 124, 1227 (2002)).
Cheng et al. (Langmuir vol. 16, 5333 (2000)) and Frankel et al. (J. Am. Chem. Soc. vol. 116, 10057 (1994)) reported that compounds consisting of single, chiral diacetylenic chains can form tubules. In addition, Singh et al. (J. Chem. Soc., Chem. Commun. vol. 18, 1222 (1988)) discussed the formation of tubules from a non-chiral amphiphile composed of two diacetylenic chains, and Lindsell et al. (Chem. Mater. vol. 12, 1572 (2000)) discussed the preparation of micrometer sized tubules from non-chiral amphiphile composed of single diacetylene chain. However, the tubule-like structures discussed in these publications were quite heterogeneous.